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Chapter 12: Problem 5

Explain as precisely as you can, but in no more than 100 words, the ionic basis of an action potential and how it is passed along an axon.

Short Answer

Expert verified
Action potential is initiated by Na鈦� influx, propagates along the axon by sequential depolarization, and resets with K鈦� efflux and sodium-potassium pump activity.

Step by step solution

01

Resting Membrane Potential

The axon membrane is polarized due to a high concentration of sodium ions (Na鈦�) outside and potassium ions (K鈦�) inside, creating a resting membrane potential of about -70 mV.
02

Depolarization

When a stimulus reaches the neuron, voltage-gated Na鈦� channels open, allowing Na鈦� to rush in, causing the inside of the membrane to become positive, thereby depolarizing the membrane.
03

Repolarization

Following depolarization, Na鈦� channels close and K鈦� channels open, allowing K鈦� to flow out of the cell. This restores the negative charge inside the membrane and returns it to its resting potential.
04

Hyperpolarization

The membrane potential becomes even more negative than the resting potential as K鈦� channels close slowly, creating a temporary hyperpolarization.
05

Action Potential Propagation

The local depolarization opens Na鈦� channels in the adjacent sections of the axon membrane, allowing the action potential to travel along the axon as a wave of depolarization.
06

Restoration of Resting State

The sodium-potassium pump restores ion balance by moving Na鈦� out and K鈦� in, maintaining the resting membrane potential and readiness for the next action potential.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Resting Membrane Potential
The resting membrane potential is the electrical potential difference across the neuron membrane when the neuron is not actively sending an impulse. Quite simply, it's like a battery that's fully charged and ready to go. This potential is typically around -70 mV and is established because of the unequal distribution of ions across the cell membrane.
The neuron has a high concentration of sodium ions (Na鈦�) outside and potassium ions (K鈦�) inside. This gradient is crucial because it prepares the neuron to respond rapidly to stimuli.
  • Inside the neuron: lots of K鈦�, a little Na鈦�
  • Outside the neuron: lots of Na鈦�, a little K鈦�
This setup is akin to a spring loaded and ready to go, but at rest.
Depolarization
Depolarization is the kickoff event of an action potential. It all starts when the neuron receives a stimulus strong enough to disrupt the resting potential. When this happens, voltage-gated sodium channels in the membrane open up. This opening allows Na鈦� ions to flood into the cell.
As these positively charged ions enter, they cause the inside of the neuron to become more positive, flipping the sign of the membrane potential. This change is what we call depolarization. This phase is essential, as it sets off the chain reaction needed for an action potential to travel along the neuron.
  • Na鈦� channels open
  • Na鈦� rushes in
  • Inside becomes positive
Repolarization
After depolarization, the cell must reset to be ready for the next signal. Repolarization is the process that does just that. With Na鈦� channels closing, the voltage-gated potassium channels get their turn to open. K鈦� ions, being more concentrated inside, now rush out of the neuron.
This exit helps to restore the inside of the neuron to a more negative charge, returning the membrane potential toward its resting state. This phase is essential because it ensures that the neuron can fire again and is ready for future signals.
  • Na鈦� channels close
  • K鈦� channels open
  • K鈦� rushes out
  • Inside becomes negative again
Sodium-Potassium Pump
The sodium-potassium pump is the unsung hero of the action potential saga. It's a crucial membrane protein that helps maintain the resting membrane potential once the action potential has passed. After repolarization, the balance of Na鈦� and K鈦� across the membrane gets a bit tripped up. This is where the sodium-potassium pump comes into play.
It uses energy to move three sodium ions out of the cell and two potassium ions back in. This exchange helps restore the ion concentration to their original, non-excited states. Basically, it ensures the neuron resets its 'battery' for the next potential firing.
  • Uses ATP for energy
  • Moves 3 Na鈦� out
  • Moves 2 K鈦� in
  • Keeps membrane potential primed

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Most popular questions from this chapter

The resting membrane potential of a typical animal cell is about \(-70 \mathrm{mV}\), and the thickness of a lipid bilayer is about \(4.5 \mathrm{nm} .\) What is the strength of the electric field across the membrane in \(\mathrm{V} / \mathrm{cm} ?\) What do you suppose would happen if you applied this field strength to two metal electrodes separated by a \(1-\mathrm{cm}\) air gap?

Name at least one similarity and at least one difference between the following (it may help to review the definitions of the terms using the Glossary): A. Symport and antiport B. Active transport and passive transport C. Membrane potential and electrochemical gradient D. Pump and transporter E. Axon and telephone wire F. Solute and ion

Acetylcholine-gated cation channels do not discriminate between \(\mathrm{Na}^{+}, \mathrm{K}^{+},\) and \(\mathrm{Ca}^{2+}\) ions, allowing all to pass through them freely. So why is it that when acetylcholine binds to this protein in the plasma membrane of muscle cells, the channel opens and there is a large net influx of primarily \(\mathrm{Na}^{+}\) ions?

In the disease myasthenia gravis, the human body makes-by mistakeantibodies to its own acetylcholine receptor molecules. These antibodies bind to and inactivate acetylcholine receptors on the plasma membrane of muscle cells. The disease leads to a devastating progressive weakening of the muscles of people affected. Early on, they may have difficulty opening their eyelids, for example, and, in an animal model of the disease, rabbits have difficulty holding their ears up. As the disease progresses, most muscles weaken, and people with myasthenia gravis have difficulty speaking and swallowing. Eventually, impaired breathing can cause death. Explain which step of muscle function is affected.

Name the three ways in which an ion channel can be gated.
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